1. Technical Field
The present invention relates to techniques for carrying out location registration to a plurality of mobile communication networks.
2. Related Art
In recent years, a number of third generation mobile communication network systems, such as W-CDMA (Wideband Code Division Multiple Access) cdma 2000(Code Division Multiple Access 2000) have been provided. Unlike second generation mobile communication network systems, such as GSM(Global System for Mobile Communications), third generation mobile communication network systems can provide such services as high-speed data communication, or international roaming. However, service areas for mobile communications of a third generation system are smaller than those for mobile communications of a second generation system; therefore, it is common for a third generation system user to register his/her own mobile terminal to a mobile communication network of a second generation system in a case that a mobile communication network of a third generation system is not available.
To overcome the above problems, Patent Publication 1(JP2002-535902 corresponding to U.S. Pat. No. 6,594,242) discloses techniques to enable a user of a mobile terminal to switch from a mobile communication network of a third generation system to that of a second generation system. A base station belonging to a mobile communication network of a third generation system transmits to a mobile terminal when the mobile terminal is within range of a base station of a mobile communication network of a second generation system, control information including channel information specifying a base station belonging to the network of the second generation system. Then, the mobile terminal specifies connects to an available base station belonging to a mobile communication network of the second generation system on the basis of the channel information included in the above control information. Accordingly, a mobile communication network to which a mobile terminal is connected is changed from a mobile communication network of a third generation system to a mobile communication network of a second generation system.
Where both second and third generation mobile communication systems are available, it is preferable to use a mobile communication service of a third generation system, since a third generation system has the above-described merits in comparison with a second generation system. In one method for using a third generation system by priority, a mobile terminal executes the following operations: A mobile terminal, standing by for a call in a mobile communication network of a second generation system, cancels its standby status periodically, and detects location information transmitted from a mobile communication network of a third generation system. Then, the mobile terminal is registered to a mobile communication network of a third generation system if such location information is detected.
However, when in the process of detecting location information transmitted from a mobile communication network of a third generation system, a user of a mobile terminal cannot answer an incoming call via the mobile communication network of the second generation system. That is to say, if the mobile terminal is registered to a mobile communication network of a third generation system without any restrictions, it is difficult to call a mobile terminal via a mobile communication network of a second generation system. Further, as compared with standing by for a call, location registration uses more electric power. From this point of view, it is not satisfactory to be registered to a mobile communication network of a third generation system without any restrictions.